The mechanism effecting oxidative stress may now be approached at the level of molecular switches operating with aryl sulfotransferase IV (EC 2.8.2.9). This sulfotransferase from rat liver has the broad substrate range that is characteristic of the enzymes of detoxication. With the standard assay substrates, 4- nitrophenol and 3?-phosphoadenosine 5? phosphosulfate, sulfation is optimum at pH 5.4 whereas the enzyme is ineffective in the physiological pH range. These properties would appear to preclude a physiological function for this cytosolic enzyme. Partial oxidation of the enzyme, however, results not only in an increase in the rate of sulfation but also in a shift of the pH optimum to the physiological pH range. The mechanism for this dependence on redox environment involves oxidation at cysteine 66, either by formation of a mixed disulfide with glutathione, or by the formation of an internal disulfide with cysteine 232. Oxidation at cysteine 66 acts only as a molecular redox switch and is not directly involved in the catalytic mechanism. Underlying the activation process is a change in the nature of the ternary complex formed between enzyme, phenol and the reaction product, adenosine 3?,5?-bisphosphate. The reduced enzyme appears to form an inhibitory, dead-end ternary complex, the stability of which is dictated by the ionization of the specific phenol substrate. The ternary complex impedes the binding of PAPS that is necessary for the initiation of a further round of the reaction, and is manifest as profound, substrate dependent inhibition. In contrast, the ternary complex formed with the oxidized form of the enzyme allows binding of PAPS and the unhindered repetition of the reaction cycle. This molecular switch is freely reversible